A communication system having an OFDM (Orthogonal Frequency Division Multiplexing) frame has been known and it has been adopted in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) of the Japanese integrated services digital broadcasting terrestrial standard. If a receiving device moves in a Rayleigh environment in which multiply scattered waves due to a multipath are received, the received frequency varies irregularly due to the Doppler effect. Because of this, fluctuations in received frequency are taken into consideration for a receiving device and in a receiving device for terrestrial digital broadcasting, demodulation processing that has taken into consideration the influence of the Doppler shift, which is a phenomenon in which the carrier frequency shifts at the time of reception while moving, is performed. A difference between the frequency of a signal and the frequency of the signal having changed due to the Doppler shift is a Doppler frequency.
In the case of a receiving device adopting the OFDM system, the influence of fluctuations in frequency due to the Doppler effect appears as fluctuations in phase in the symbol direction. Because of this, in OFDM propagation path estimation, interpolation is performed while taking into consideration the fluctuations in phase in the symbol direction. As a symbol interpolation coefficient, a flat coefficient is selected so that it is possible to perform reception robust to noise in the case where the Doppler frequency is small, i.e., in the case where fluctuations due to movement are small. However, in this case, there is such a problem that reception is vulnerable to fluctuations in phase. On the other hand, in the case where the Doppler frequency is large, i.e., in the case where fluctuations due to movement are large, a large weight is given to the center of the symbol interpolation coefficient so that reception is robust to fluctuations in phase. However, in this case, there is a problem that reception is vulnerable to noise.
It is desirable to select an optimum symbol interpolation coefficient in accordance with the Doppler frequency as described above, and in order to make such a selection, the Doppler frequency is estimated.
Into the OFDM frame of ISDB-T of the Japanese integrated services digital broadcasting terrestrial standard, SP (Scattered Pilot) signals are inserted at regular intervals and the pilot signals are arranged at 12-carrier intervals in the subcarrier (frequency) direction, which is the horizontal axis, and in the symbol (time) direction, the pilot signals are arranged so as to shift every three subcarriers between two successive symbols. There is a known technique hitherto, which estimates the Doppler frequency applied to the carrier frequency by reception while moving using pilot signals extracted from a received signal after a Fourier transform.
As a method for estimating the Doppler frequency from the OFDM signal, there are known a first method that makes use of pilot signals with different subcarrier numbers and a second method that makes use of pilot signals with the same subcarrier number.
In the second method, it is common to find a phase rotation amount of two pilot signals with the same subcarrier number of the received signals of different symbols and then, to estimate the Doppler frequency therefrom. In the case where the second method is performed in the OFDM frame configuration, such as that in the terrestrial digital method, a difference in phase between the pilot signal of the current symbol and that of the symbol four symbols ahead thereof is found and the phase rotation amount is found therefrom. However, by the second method, if the Doppler frequency is large and the phase rotation amount exceeds ±180 degrees, it is no longer possible to find an accurate phase rotation amount. Because of this, there has been the problem that it is not possible to estimate an accurate Doppler frequency by the second method in the case where the Doppler frequency is large.
In the first method, the Doppler frequency is found based on the phase rotation amount of two pilot signals with different subcarrier numbers of the received signals of different symbols. First, impulse responses are found by performing an inverse Fourier transform on the two different pilot signals, respectively. Then, peak positions of the impulse responses are found and the phase rotation amount of the two pilot signals at the peak positions is found. Then, the amounts of frequency shift of both the pilot signals and the phase correction amounts derived from the delay amount at the peak positions are added. The Doppler frequency is estimated from the corrected phase rotation amount. In the first method, it is possible to use pilot signals with different subcarrier numbers, and therefore, it is possible to find a phase rotation amount less than four symbols and to obtain an accurate Doppler frequency even in the case where the Doppler frequency is large compared to that in the second method. Because of this, in the normal case, it is desirable to obtain a Doppler frequency by applying the first method rather than the second method.